1. Field
The present invention relates to an imaging device including a photodiode. In particular, the present invention relates to a technique for improving detection sensitivity of the photodiode without increasing dark current noise.
2. Description of the Related Art
An imaging device such as an active pixel sensor (hereinafter also referred to as an APS) has a plurality of pixels each including a photodiode. Light incident on the pixel is converted into optical carriers by the photodiode and outputted as an electrical signal to generate an image signal. Generally, the imaging device is formed using pixels called a three-transistor type and a four-transistor type (Japanese Unexamined Patent Application Publication No. 2000-312024, Japanese Unexamined Patent Application Publication No. Hei 10-209422). In this kind of pixel, in order to prevent a dark current, at a surface of a diffusion layer to store optical carriers, a diffusion layer of an opposite conductive type with a higher impurity concentration than that of the above diffusion layer is formed. The dark current is a current flowing when no light is incident and a leak current due to semiconductor crystal defects. By forming the diffusion layer of the opposite conductive type on the diffusion layer to store the optical carriers, the occurrence of crystal defects in the diffusion layer to store the optical carriers can be prevented.
However, although the above diffusion layer of the opposite conductive type can prevent dark current noise, it makes a depletion layer of the diffusion layer to store the optical carriers relatively thinner. Therefore, light detection sensitivity in the pixel lowers due to the formation of the diffusion layer of the opposite conductive type. The dark current noise is large near an isolation region and in a contact region to draw out the electrical signal and so on and small in the other regions. Conventionally, any proposal to minimize the influence of the dark current noise and improve the light detection sensitivity has not been made.